68 Baculum [including the marine otter, Lontra felina (undescribed but pre- sumed) and the sea otter, Enhydra lutris ], and Pinnipedia. The bac- ulum is absent in Cetacea and Sirenia. The corresponding element in females is the little-studied clitoris bone (os clitoridis), which has been documented for polar bears and several pinniped species, but presumably is present in all pinnipeds, and in marine and sea otters B (it is present in the northern river otter, Lontra canadensis ; Mohr, 1963 ; Fay, 1982 ). The baculum is one of several so-called heterotopic bones in B mammals, like the kneecap (patella), which form through ossifi ca- tion in connective tissue. In rodents, the bacular shaft is true bone, and includes hemopoietic tissue in the enlarged basal portion. In the caniform Carnivora (which includes bears, otters, and pinnipeds) bacular development has been detailed only in the dog (Canis famil- iaris ) but is probably similar in other Caniformia. The dog baculum Baculum develops in the proximal portion of the penis, in association with the fi brous septum between the paired corpora cavernosa penis, or EDWARD H. MILLER in their fi brous non-cavernous portion; centers of ossifi cation on left and right sides fuse early in development. The developing baculum he baculum (os penis) is a bone in the penis that occurs grows dorsally above the urethra, and thickens. The bacular base in small insectivorous placentals (orders Afrosoricida, becomes fi rmly attached to the corpora cavernosa and to the fi brous TErinaceomorpha, and Soricomorpha), Chiroptera, Primates, tunica albuginea which surrounds them. Rodentia, and Carnivora ( Burt, 1960 ). Among marine mammals, it The urethral groove in the baculum is deep in the dog but is shal- is present in Ursidae (polar bear, Ursus maritimus ), all Mustelidae low to absent in bacula of marine mammals ( Fig. 1A lower, 1B lower ), Figure 1 Bacula of marine mammals are large, but most are morphologically simple: (A) polar bear (Ursus maritimus ); (B) subantarctic fur seal ( Arctocephalus gazella); (C) Mediterranean monk seal (Monachus monachus); (D) crabeater seal (Lobodon carcinophagus); (E) Weddell seal (Leptonychotes weddellii ). All scale bars, 5 cm (no scale bars for E2, E3). Bacula in (A)–(D) are shown in right lateral (upper) and ventral (lower) views. E1: Baculum in right lateral view (note cross- sectional shapes at the indicated points). E2: Oblique view (right side) of the bacular apex (same specimen); dashed line indi- cates how much growth occurs in the crest (above the line), following sexual maturity. E3: Apical view (dorsal surface above; same specimen). A from R. Didier (1950; Mammalia 14 , 78–94); B from R. Didier (1952; Mammalia 16, 228–231); C from P. J. H. van Bree (1994; Mammalia 16, 228–231); D from R. Didier (1953; Mammalia 17, 21–26); E from G. V. Morejohn (2001; Journal of Mammalogy 81 , 877–881). Baculum 69 3mm 4mm 5mm although is likely present terminally in the undescribed baculum of the marine otter, because this is the pattern in the northern river otter ( Baryshnikov et al ., 2003 ). Bacula of polar bears and phocid seals are fairly simple, being more or less straight or slightly curved (arched dorsally) structures, and lacking elaborate apices ( Fig. 1 ). In at least some phocids, the bacular apex has a prominent cartilagi- nous cap (e.g., hooded seal, Cystophora cristata ). Cross-sectional B shapes of phocid bacula vary considerably among species, and a prominent crest develops on the anterior dorsal surface in some Antarctic seals ( Fig. 1E ). The bacular apex is larger and more elab- orate in otariids than phocids, in keeping with the close proximity of the apex to (beneath) the glans penis in otariids where apical size and shape may be functionally important during copulation ( Fig. 2 ). (A) (B) (C) Mustelids possess some of the most diverse and morphologically elaborate elaborate bacula within the Caniformia, although that of the sea otter is relatively simple ( Fig. 3 ; Baryshnikov et al ., 2003 ). Within species, bacula are variable in size, shape, cross-section, and specifi c structural features, even among individuals of the same age. For exam- ple, a dorsal keel may be present or absent in southern elephant seals (Mirounga leonina); processes on the shaft near the apex are variably present in California sea lions ( Zalophus californianus ); and bacula may be bilaterally asymmetrical or slightly twisted ( Fig. 1D). Bacula of Carnivora are fairly large (Dixson, 1995 ; Lariviére and Ferguson, 2002 ; Ramm, 2007 ). Bacular length is approximately 6% of body length in otariids, but relatively longer in polar bears ( ϳ 8%) and phocids (8% in hooded seals; 10% in harp seals, Pagophilus groen- (D) (E) landicus ); the baculum is also much thicker in phocids than otariids ( Mohr, 1963 ; Scheffer and Kenyon, 1963 ). In pinnipeds, and indeed 10 mm 10 mm among all mammals, the walrus ( Odobenus rosmarus ) has the largest baculum both absolutely (to 62.4 cm in length and 1040 g in mass) and Figure 2 The bacular apex is morphologically complex and inter- relatively (18% of body length; Fay, 1982 ). Interspecifi c differences in specifi cally diverse in Otariidae. The apex is shown in apical view bacular size in mammals have been linked to diverse selective pres- (dorsal surface up) for (A) unknown species of Arctocephalus fur sures: reproductive isolation between species; aquatic vs terrestrial seal; (B) northern fur seal ( Callorhinus ursinus ); (C) California sea copulation; copulatory duration or pattern; sexual selection and mat- lion ( Zalophus californianus ); (D) Australian sea lion ( Neophoca ing system; climate; and risk of fracture ( Scheffer and Kenyon, 1963 ; cinerea ); and (E) Hooker’s sea lion ( Phocarctos hookeri ). From Eberhard, 1985 ; Dixson, 1995; Lariviére and Ferguson, 2002; Ramm, G. V. Morejohn (1975; Rapports et Proces-verbaux des Reunions, 2007 ). Fractures result from accidents (e.g., falls in walruses), sudden Conseil International pour l’Exploration de la Mer 169 , 49–56). movements during intromission (e.g., in aquatically mating Caspian Figure 3 The baculum of the sea otter ( Enhydra lutris ) is fairly simple, except for the apex (to the right). Top: dorsal view; center, ventral view; bottom, right lateral view. Scale is in centimeters. From K. W. Kenyon (1969; North American Fauna 68 , 1–352). 70 Baculum B Figure 4 Developmental changes in bacular size and shape, illustrated by repre- sentative specimens from northern fur seals ( Callorhinus ursinus ), ranging in age from newborn (left) to 8 years of age (right). Specimens are shown in right lateral view, with bacular apex at the top. Scale is in centimeters. From V. B. Scheffer (1950; Journal of Mammalogy 31 , 384–394). seals, Pusa caspica), and aggressive social interactions (e.g., fi ghts in adult male sea otters). Healed fractured bacula have been docu- mented for several species. Bacula likely serve several functions: as a mechanical aid in copulation (especially in the absence of full erec- tion), or maintenance of intromission, in aquatic copulations; and to initiate or engage neural or endocrinological responses in females. Bacular size may be limited by adverse effects on females: a female sea otter and a harbor seal ( Phoca vitulina) pup died from perforation of the vagina during forced copulations with male sea otters. Bacular form and diversity refl ect multiple functions, and hence likely have multiple adaptive explanations within and across species. In Carnivora, bacula grow throughout life in thickness and mass (particularly at the proximal or basal end), but not in length ( Fig. 4 ). Bacular growth is most rapid around puberty. Differential growth occurs in different parts of the baculum (e.g., bacular apex, shaft, and base, in Steller’s sea lion, Eumetopias jubatus ; Miller et al ., 2000 ). The baculum is anatomically complex and species-specifi c in many groups, so has been used extensively in mammalian system- atics. In addition, bacular growth has been investigated in furbear- Figure 5 Genitals of African fur seal ( Arctocephalus p. pusillus) ers and game animals, because it can be informative about age and drying under a work table at a seal processing facility in Luderitz, time of puberty. More recently, the baculum has been studied in the South Africa (1994). Photo: ©International Fund for Animal context of mate-choice and sexual-selection theories. In Alaska, the Welfare. Baiji 71 Ramm , S. A. ( 2007 ). Sexual selection and genital evolution in mammals: A phylogenetic analysis of baculum length. Am. Nat. 169 , 360 – 369 . Scheffer , V. B. , and Kenyon , K. W. ( 1963 ). Baculum size in pinnipeds. Z. Säugetierkund. 28 , 3 8 – 4 1 . B Baiji Lipotes vexillifer KAIYA ZHOU I. Characteristics and Taxonomy he baiji or Yangtze river dolphin is endemic to the middle and lower reaches of the Yangtze River in China. It is a relict Tspecies and the only living representative of a whole family of mammals. It was described early in the ancient dictionary, Erh Ya , published as long ago as 200 bc . The baiji is a graceful animal with a very long, narrow and slightly Figure 6 Cooked seal genitals prepared as a meal in the upturned beak. It can be easily identifi ed by the rounded melon, lon- Guolizhuang Penis Restaurant, Beijing, China (September 7, 2007). gitudinally oval blow hole, very small eyes, low triangular dorsal fi n, These were advertised as Canadian seal, so probably were from a harp and broad rounded fl ippers ( Fig. 1 ). The color is generally bluish gray seal ( Pagophilus groenlandicus ), and killed in the commercial hunt in or gray above and white or ashy white below. Females are larger than Quebec or Newfoundland and Labrador. Photo: Feng Li/Gettyimages. males. Maximum recorded length for females is 253 cm and for males is 229 cm ( Zhou, 1989 ). Signifi cant differences between the sexes in U.S. Fish & Wildlife Service requires that hunters leave the baculum external proportions were demonstrated in nine characters, and the attached to the hide of sea otters and polar bears, to confi rm sex.